In the field of adaptive optics, in which a deformable mirror is adjusted in order to maintain the quality of an output beam in order to compensate for thermal or other distortions, a great deal of work has been done in sensing the quality of an output beam and adjusting the deformable mirror in order to produce the output beam with a desired configuration. A standard tool has been the Hartman sensor in which a number of lenses focus small samples of the beam onto a corresponding number of detectors, typically an array of quadrant optical detectors. Comparison of the sample beam with a reference beam permits a deformable or adjustable mirror to be adjusted in order to align the wavefront of the output beam correctly. A convenient reference on Hartmann sensors is contained in D. Malacara, "Hartmann Test of Aspherical Mirrors", APPLIED OPTICS, Volume II, Page 99, 1972.
The art of adaptive optics has not yet solved the "piston" problem in which a number of separate beams are combined on a large mirror which itself is made up from a number of smaller mirror elements. In order to produce a high quality output beam, the separate mirror elements must be aligned so that the output beam has the same tilt as the reference beam and also so that the path lengths through the system that strike each separate mirror element are exactly equal for all portions of the mirror. The situation in which the several sub-beams are correctly aligned in angle but have phases that differ is known as the piston problem.